Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved front assembly for color tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional curved shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate, and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. When heated, the tension mask distorts in a manner quite different from that of the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen with its deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may be, by way of example, about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance". As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow masks mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension; e.g., 30 pounds per linear inch. In consequence, the support means must be of high strength so the mask is held immovable; an inward movement of the mask of as little as 0.0002 inch can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal contraction compatible with that of the glass, and by its composition, be bondable to glass. Also, the support means must be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the support structure is composed must be adaptable to machining or to other forms of shaping so the structure can be contoured into near-perfect flatness. Otherwise, voids will exist between the metal of the mask and the support structure, preventing positive, uniform contact of the mask to the support structure necessary for proper mask securement.
Means for securing the shadow mask support to the inner surface of the faceplate may comprise a cement in the form of a solder glass, also known as "frit." While satisfactory in the main, cement of this type has a significant disadvantage in that it tends to create pockets in which screening fluids may lodge and be released later as contaminants. The cathodes of the electron gun are particularly susceptible to poisoning from contaminants. Also, deep-lying ones of such pockets in the devitrified solder glass may be connected to the surface of the solder glass by a tiny conduit. The air retained in the pocket may not be depleted during the exhaust cycle, but slowly leak out through the conduit after the tube is sealed, resulting in a reject as a "gassy" tube.
In U. S. Pat. No. 3,894,321 to Moore, of common ownership herewith, there is disclosed means for mounting a foil shadow mask on "rails" which extend from the faceplate. In another embodiment, the faceplate is shown as having an inner ledge that forms a continuous path around the tube, the top surface of which is a Q-distance away from the faceplate for receiving a foil mask.
In U.S. Pat. No. 4,695,761 to Fendley, of common ownership herewith, a hollow foil shadow mask support structure is disclosed in which the structure has a cross section in the form of an inverted "V," the narrow end of which provides for receiving the mask, and wherein the wide, open end is secured to the faceplate. The means of securement is by beads of glass frit. Other foil mask support structure embodiments are also disclosed, such as a hollow tube and a rectangle.
In referent application Ser. No. 942,336 of common ownership herewith, there is disclosed a support structure for a tensed foil shadow mask comprising an inverted channel member of metal with a stiffening core of a material such as ceramic secured within, and lateral to, the channel member. In one embodiment of the invention, the space between the stiffening core and the inner walls of the channel is filled with a devitrified glass frit. In another embodiment, a ceramic slurry is poured into a V-shaped support member and is allowed to set, with the object of stiffening the support structure. Except for the area of the structure that contacts the faceplate, the ceramic is totally enclosed within the support structure. A devitrified glass frit may be used as the stiffening material, similarly enclosed within the structure.
In application Ser. No. 178,175 to Capek et al, also of common ownership herewith, there is disclosed a support structure comprising a hollow shell filled with at least two devitrified solder glass compositions each having a different viscosity when in the form of an undevitrified solder glass paste. The object of the invention is to prevent the formation of the previously described air-filled pockets in the devitrified solder glass. The solder glass is completely enclosed within the support structure.